This invention relates to bicycle shifting systems and more particularly to a semi-automatic shifting system for a bicycle that includes an automatically actuated internally geared mechanism and a manually actuated derailleur.
It is known in the art relating to bicycles to provide variable speed transmissions in order to accommodate different riding conditions as well as the comfort of different riders. Conventional bicycle transmissions may be divided into generally two types: transmissions that use an internally geared mechanism and transmissions that use an externally geared mechanism. The externally geared mechanism may include a derailleur that shifts a drive train coupled to a drive wheel and an internally geared mechanism may be an internal gear hub system coupling a hub shaft to a drive wheel.
A derailleur transmission will usually have two sets of sprockets or gears, two to three sprockets at the pedal-crank assembly or front gears and five to nine sprockets at the rear wheel of the bicycle or rear gears. When the gears are shifted manually under the control of the rider, the rider can independently select a front gear and a rear gear. By selecting appropriate front and rear gears, a rider determines a gear ratio that is suitable for the riding conditions; For example, a rider may select a low gear ratio when traveling uphill at a low rate of speed and a high gear ratio when traveling downhill at a high rate of speed. The gear ratio specifies the number of revolutions that the rear wheel turns per revolution of the pedal crank assembly. Each combination of a front and rear gear is referred to as a bicycle gear. The front gears usually have wide differences in number of teeth, resulting in large changes in gear ratios when shifting the front. The rear gears usually have small differences in number of teeth, resulting in small changes in gear ratios when shifting the rear. Therefore, front shifting is referred to as a macroadjust to the gear ratio and rear shifting is a microadjust to the gear ratio.
An internally geared transmission is usually located in the rear wheel hub. It basically makes use of a planetary gear mechanism to provide a plurality of gears. When an internal hub has only a few gears such as two to four gears, the changes in the gear ratios are large, much like the macroadjustments of the front gears. The internal hub may be used in combination with a derailleur that usually has 5 to 9 speeds, providing microadjustments to the gear ratio.
Traditionally, these types of shifting systems have been operated manually by the rider, although automatic shifting has been attempted using both derailleurs and internal hubs. However, there are several drawbacks to both manual and automatic shifting systems. With a manual derailleur shifting system having front and rear gears, the rider must operate two independent shifters to find the appropriate gear. The rider must avoid cross chaining, a condition which is less efficient. Also, the rider must anticipate changing riding conditions ahead. If the rider comes to an abrupt stop, he must first downshift or risk starting again in a high gear. Lastly, shifting cannot be done at a standstill or without pedaling.
When using an automatic derailleur system the derailleur cannot be moved or shifted when the rider is not pedaling. Further, during automatic shifting of the derailleur, the bicycle chain can unexpectedly slip or come off a sprocket, resulting in the rider losing control or balance because he could not anticipate the shifting of the chain.
An automatic internal gear hub solves several problems associated with a derailleur system. One advantage is that the rider does not have to pedal for the internal hub to shift. For example, it will shift down while the rider is coasting to a stop. Thus when the rider resumes pedaling he will start in the appropriate gear. Another advantage with the internal hub is that there is generally no slippage of the chain, resulting in a safer ride and smoother shifts.
Although an automatic internal gear hub system is in many ways better than a derailleur system, it has several drawbacks of its own. One drawback associated with automatic internal gear hub systems is the generally small overall gear range it provides. Typically, automatic internal hubs have only three to four gears. This is a drawback because the feel of the ride is determined by three factors, the starting gear, the speed at which shifting occurs, and the final gear the rider is in when the last shift point has been reached. For example, the rider must start off in the lowest gear and then a shift or two occurs when the wheel speed reaches certain predetermined shift points. After the last shift point has been reached, the rider remains in the same high gear without any means to change his pedaling rate. The automatic internal hub fails to provide the rider with the ability to fine-tune his pedaling rate. Therefore, there is a need for a shifting system that provides the multi-gear range of a derailleur system and the automatic features of an internal gear hub system.
The present invention provides a semi-automatic gear change system mountable about a drive wheel axle of a bicycle. The system includes a drive wheel sprocket assembly that includes a plurality of sprockets having different number of teeth. The sprocket assembly is coupled to the drive wheel axle. A derailleur is operable to entrain a drive chain onto a selected one of the sprockets. A manually actuated shifter is coupled to the derailleur to select one of the sprockets. An internally geared mechanism couples the hub shaft to the drive wheel and has a plurality of transmission modes. An automatic shift actuator is coupled to the internally geared mechanism to actuate a shift between one of the transmission modes and another of the transmission modes based on a plurality of predetermined shift points. The shift points may be based on wheel speed.
In one embodiment of the present invention, the internally geared mechanism is an internal gear hub system and the drive wheel is the rear wheel of the bicycle. Preferably, the internal gear hub system is a three-speed hub and the rear derailleur is a seven-speed derailleur. With this configuration, the rider only has to operate one shifter while still having the benefits of a wide range of gears. The rider only controls the fine-tuning, while the automatic portion of the shifting system selects among the larger gear ranges.
In another embodiment of the present invention, the gear change system further includes a wheel speed sensor for sensing wheel speed and generating a speed signal indicative of the sensed wheel speed. The automatically actuated shifter includes a memory for storing the predetermined shift points which relate each transmission mode to a wheel speed and a controller for receiving the wheel speed signal and controlling the internally geared mechanism such that when the wheel speed signal substantially equals the stored wheel speed, the internally geared mechanism is shifted into the transmission mode related to the stored wheel speed.
One advantage of having an automatic hub and a manual derailleur as disclosed in the present invention is that at any given wheel speed, the rider can maintain the same speed but change his/her pedaling rate by manually adjusting the derailleur. Since the rider is directly changing the gearing, the change in cadence is immediate. In contrast, the changing of shift points in a stand-alone automatic internal hub does not result in immediate changes of the pedal speed. The rider may still have to decrease the bicycle speed to cause a shift to a lower gear.
Another advantage of the present invention is that upon an abrupt stop, the automatically operated internal hub will shift the rider to a low gear without the rider having to pedal. Thus, when the rider resumes pedaling the rider will be in the appropriate starting gear. However, if the rider is starting on uphill or downhill sloped terrain, the manual derailleur allows the rider to fine-tune the gear ratio for the given condition.
The present invention also provides a method of shifting a bicycle. The method includes the steps of automatically shifting an internally geared mechanism having a plurality of transmission modes coupling a drive shaft to a drive wheel in accordance with at least one bicycle condition and a manually shifting a derailleur to shift a drive chain coupled to the drive wheel. Preferably, the bicycle condition is wheel speed and the drive wheel is the rear wheel of the bicycle.
These and other features and advantages of the invention will be more fully understood from the following description of certain specific embodiments of the invention taken together with the accompanying drawings.